Popular Woodworking 2007-12 № 166, страница 55
BY ROBERT W. LANG
Arc Drawing Bow
Searching for (and Finding) the Arc of the Competent.
hen woodworking throws you a curve, how do you respond? It's easy to become locked into one solution because it's familiar and miss an e asy alternative. This is often the case when a curve is part of a design. Laying out and cutting arcs is a common task, but the best way to do it is a matter of scale. Different tactics are needed for larger or smaller radii.
If the radius of a curve is less than 6" or so, a compass is the obvious choice. If the radius is known, it's simply a matter of setting the compass point to the right dimension and swinging the arc. But many times, particularly if you're creating a new design, you don't know (and likely don't care) what the radius is. You want the arc to stop and start at certain points along an edge, and you want the high point to be a certain distance from the edge.
If this is this case, here is a simple way to find the radius you want, using your layout tools to find it. It is easier to do this on a piece of graph paper or a scrap of plywood rather than on an actual workpiece, because the point of the compass will end up off the edge of the piece.
Start with a horizontal line representing the edge. In most cases, such as a table apron, the curve should start and stop a V2" or so from the end of the board. This gives you more control of the intersection, and if you round off the point where the short straight line meets the arc, it will look like a continuous curve.
At the center of the horizontal line, make a vertical line with a mark at the rise of the arc. Scratch your head and feel some regret about sleeping through high-school geometry. Snap out of that, then pick up your compass and a straightedge.
Set the compass to the distance between an end of the arc and the high point, and swing an arc above and below from each end, ending up with two curved Xs. It might make you feel
76 ■ Popular Woodworking December 2007
The important part. When drawing an arc, where it stops and starts, and the high point in between are more important than knowing the exact radius. Connect those points and you're ready to go.
better to do the same thing from the other end of the arc too, but you don't really need to.
Place the straightedge across the intersections of the Xs and draw a line below your
the center vertical line is the center of your arc. If you drew from the other side as well, two angled lines and the center line should all intersect.
arc-to-be. The point where this line meets Put the point of the compass on the inter-
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OPENING PHOTO BY AL PARRISH